The volume flow is defined as the volume of fluid flowing through the cross section of the measuring tube per unit of time and the mass flow is defined as the mass of fluid flowing through the cross section of the measuring tube per unit of time.
It is known that, during the operation of a vortex flow pickup of this type, a Karman vortex street is produced downstream of the bluff body and its pressure fluctuations are converted by a vortex sensor into an electrical signal, the frequency of which is proportional to the volume flow or the flow velocity.
In U.S. Pat. No. 6,003,384 there is a description of a currently customary vortex flow pickup for measuring the volume flow or the flow velocity of a fluid which is flowing in a direction of flow in a measuring tube having a tube wall, which vortex flow pickup comprises:                a bluff body which is arranged along a diameter of the measuring tube and                    serves for producing Karman vortices and            is connected to the tube wall of the measuring tube from the inside at a first and a second fixing location, which lie opposite each other,                        a vortex sensor, which responds to pressure fluctuations produced by the vortices, is fitted downstream of the bluff body in a bore of the tube wall of the measuring tube and seals off this bore,        the center of the bore lying together with the center of the first fixing location of the bluff body on a generatrix of the measuring tube and        the vortex sensor comprising:                    a diaphragm covering the bore, with a first surface facing the fluid and a second surface facing away from the fluid,            a wedge-shaped sensor vane, which is fastened on the first surface of the diaphragm and                            is shorter than the diameter of the measuring tube and                has principal surfaces in line with the generatrix of the measuring tube and also a front edge, and                                    a sensor element fastened on the second surface.                        
If the temperature of the fluid is also measured by means of a temperature sensor, the mass flow can be determined, for example calculated by means of a microprocessor, from the volume flow, the type of fluid and its properties as well as the temperature at any given time.
This has already been described some time ago in the case of vortex flow pickups with different types of vortex sensors. For instance, U.S. Pat. Nos. 4,048,854 and 4,404,858 each show a temperature sensor which is arranged on the tube wall of the measuring tube from the inside in such a way that it is skimmed over by the flowing fluid.
In JP-A 2000-2567 there is a description of a vortex flow pickup for measuring the mass flow, the volume flow or the flow velocity of a fluid which is flowing in a direction of flow in a measuring tube having a tube wall, which vortex flow pickup comprises                a blade which is fixed on one side to the tube wall from the inside by means of a base plate and                    during operation produces Karman vortices,            is shorter than a diameter of the measuring tube and            has parallel principal surfaces aligned perpendicularly to the direction of flow and a rounded front face,                            on which a temperature sensor is arranged,                                                first sensor elements, fastened in the vicinity of the fixing location, for pressure fluctuations of the flowing fluid produced by the Karman vortices and        second sensor elements, fastened in the vicinity of the fixing location, for deflections of the blade produced by the flowing fluid.        
This temperature sensor is also skimmed over by the flowing fluid and, as the inventors have found, is consequently not resistant to all fluids encountered in operation, i.e. some fluids corrode temperature sensors arranged in such a way.
These fluids which corrode the temperature sensor must therefore be banned from use with the vortex flow pickup by the manufacturer of the latter. However, such a ban restricts the use of these vortex flow pickups, that is the universality of their applications, and consequently also their attractiveness on the market.